Brachial Plexus Anatomy — FRCA Final MCQ

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HardRegional AnaesthesiaBrachial Plexus AnatomyFRCA Final

A 60-year-old woman undergoes an infraclavicular brachial plexus block for distal radius fracture fixation. On assessment, elbow flexion and wrist and finger extension are weak. Pinprick sensation over the little finger remains normal, as do finger abduction and adduction. Which pattern of cord sparing most likely accounts for these findings?

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Correct answer: C — Medial cord only

Explanation lettering: B = shown as A · A = shown as B

The medial cord is spared. Finger abduction and adduction are mediated by the dorsal and palmar interossei, respectively, supplied by the deep branch of the ulnar nerve. Preserved little-finger sensation also supports intact ulnar function. The ulnar nerve is normally a terminal branch of the medial cord. Conversely, weak elbow flexion indicates blockade of the musculocutaneous nerve from the lateral cord, while weak wrist and finger extension indicates blockade of the radial nerve from the posterior cord. Therefore, the lateral and posterior cords are functioning as blocked, excluding options A, B and E. Option D is incompatible with preserved ulnar sensory and motor function. Anatomical variants exist, but the ulnar nerve arises from the medial cord in approximately 98% of upper limbs.

Reference: A meta-analysis on the anatomical variability of the brachial plexus: Part III — Branching of the infraclavicular part, 2022. https://pubmed.ncbi.nlm.nih.gov/35787441/